Can a channel improve in every segment yet look worse overall?

Can a channel improve in every segment yet look worse overall?

The question: a channel's conversion rate rose in mobile, rose in desktop, rose in tablet — but its blended rate fell. Your reporting says the channel got worse. Every segment says it got better. Who's lying?

What's at work: nobody is. This is Simpson's paradox — when a trend present in subgroups reverses or vanishes after aggregation, because the mix of subgroups shifted. If the channel's traffic moved toward a lower-converting segment (say, more cold prospecting, less warm retargeting), the blended average can drop even as every component rises. The aggregate is a weighted average, and the weights moved.

Why it matters for attribution specifically: attribution reports live and die on aggregated rates — conversions per channel, ROI per source. Any time the underlying composition of a channel's traffic changes (new geos, new placements, new audience mix), the headline number can move in the opposite direction from reality. Acting on the aggregate without checking the mix is how teams cut channels that are genuinely improving.

The nuance: the paradox cuts both ways — a channel can look like it's improving in aggregate purely because its mix shifted toward easy converters, masking a real decline in every segment.

What to actually do: never read a channel's blended rate without a composition check. Decompose into stable segments and look at within-segment trends and the mix shift separately. If they disagree, trust the segments and investigate the mix.

Bottom line for practitioners: aggregated attribution numbers can flip the truth. When the total contradicts every segment, the total is an artifact of the mix — not a finding.